This invention relates to a passive fibre optic transmission system wherein optical power from a single source location, e.g. a semiconductor laser, can be distributed to a number of destination points. Typically the fibre optic system can resemble a tree structure in which power from the source location is fed, via couplers and intermediate lengths of optical fibre, from a single input fibre to a number of output branches. Such a network can be used, for example, to distribute television signals from a local transmitter to a number of domestic receivers.
Since the optical power from the source is divided between the destination points, for a given receiver sensitivity the number of such destination points which can be served by a single source is limited by the power launched into the fibre by the source device. In many instances, such as in a television distribution system, the source power must be restricted on safety grounds because of the danger of permanent damage to the human eye arising from overexposure to the invisible radiation, for example arising from looking at the end of a broken fibre.